What the activated sludge process removes
The activated sludge process is the biological stage used on most municipal sewage and much industrial wastewater. Three parts carry it: an aeration tank, where the flow is mixed with a suspended culture of micro-organisms; a clarifier, where that culture settles out of the treated water; and a return line that carries the settled sludge back to the tank.
That population does two jobs: it oxidises the biodegradable organic matter the BOD and COD figures describe, and, where the sludge is held long enough, it oxidises ammonia to nitrate. Nitrification is what makes the sludge age a design decision rather than a housekeeping figure. A third job, turning nitrate into nitrogen gas, needs a zone with no dissolved oxygen, and is covered on the A²/O article.
The four decisions below are the design dials on that culture, and they are not independent: each writes the same mass balance, which is why fixing one in isolation produces a tank that is too small, under-aerated, or unable to hold on to its own solids.
Decision one: sludge age in wastewater treatment
The sludge age is the average time a particle of solids spends in the system: the mass of solids held in the aeration tank and the clarifier divided by the mass taken out each day, in days. It is also called the solids retention time, or SRT.
It is the dial that decides most of the rest. A short age keeps the culture young and fast-growing: more excess sludge, and no time to establish the slower-growing organisms. Nitrifiers grow more slowly than the organisms that remove organic matter, so a plant that has to nitrify must hold its sludge long enough that they are retained rather than washed out; the age needed rises as the water gets colder.
Commonly quoted figures put conventional activated sludge at roughly 5 to 15 days and extended aeration at roughly 20 to 30 days. Those ranges describe common practice rather than your water, and should be read as a starting method rather than as a specification. The age a plant needs follows from the effluent requirement — whether ammonia has to be oxidised, and at what winter temperature — and from how well the sludge settles.
How to determine it: work back from the discharge standard to the slowest organism the plant has to retain at the coldest water it will see, then confirm it on the running plant, where the mixed liquor solids and the sludge wasted each day give the age actually being run.
Decision two: the food to microorganism ratio in wastewater treatment
The food to microorganism ratio, written F/M almost everywhere, is the mass of food arriving each day divided by the mass of organisms kept to consume it: BOD, occasionally COD, over mixed liquor suspended solids, in kilograms of BOD per kilogram of MLSS per day.
F/M and sludge age are the same dial seen from two sides: hold more sludge against the same load and the ratio falls, waste more sludge and it rises. A high ratio favours rapid growth, a bulky floc and more excess sludge; a low ratio means food is scarce, growth is slower, the culture is steadier and nitrification is easier to hold, until the floc becomes pin-point and slow to settle.
Starting figures often quoted for a conventional plant sit around 0.2 to 0.4 kg BOD per kg MLSS per day, with extended aeration lower, of the order of 0.05 to 0.15. Read them as a starting method rather than as a specification: the load in the ratio is your influent, not a typical one.
How to determine it: the ratio is rarely a set point in its own right. It is calculated from the design load and the solids concentration the tank will hold, and checked from the operating record — influent flow and BOD with the mixed liquor suspended solids. Where the influent analysis is uncertain, the design influent quality chart sets out which figures are worth trusting.
Decision three: the return activated sludge rate
The return rate is the flow of settled sludge pumped from the bottom of the clarifier back to the aeration tank, usually stated as a ratio to the incoming flow. Without it, the organisms that do the work leave with the waste sludge and the population in the aeration tank falls until the process fails.
The rate sets two things at once: the mixed liquor suspended solids the aeration tank runs at, because the sludge returned into a given volume sets that figure, and the hydraulic load on the clarifier, because every litre returned passes through it twice. Return too little and the solids blanket rises until solids leave with the effluent; too much and the clarifier is overloaded, which carries the blanket out just as effectively.
The rate follows from the solids concentration the tank needs and the concentration of the sludge being returned, and the second is a property of the sludge rather than of the pump. Settleability is measured as the sludge volume index, and a sludge that settles well can be returned at a lower ratio. Ratios of the order of 0.5 to 1 times the influent flow are commonly quoted for a conventional plant, and the ratio has to follow the sludge the plant actually grows.
One line worth drawing here: the return sludge line is not the internal recycle that carries nitrified liquor to an anoxic zone. Both are called recycle, and they do opposite things — one returns the culture, the other returns nitrate.
Decision four: the aeration tank in a wastewater treatment plant
The aeration tank is where the first three decisions become concrete. Its volume is the mass of solids the plant has to hold, which the sludge age and the load decide, divided by the mixed liquor suspended solids concentration the tank will run at. Two consequences follow that otherwise look like rules of thumb: a higher mixed liquor concentration lets the same load be treated in a smaller tank, and a longer sludge age at a fixed concentration needs a larger one.
Volume is only half the decision. The tank also has to deliver oxygen, and the demand has two parts: the carbonaceous part, which follows the organic load actually oxidised, and the nitrogenous part, which appears when the plant nitrifies. Oxidising ammonia to nitrate consumes a fixed quantity of oxygen per kilogram of nitrogen, of the order of 4.57 kg. A plant asked to nitrify therefore needs more air than carbon removal alone would.
Commonly quoted mixed liquor figures are of the order of 2,000 to 4,000 mg/L for conventional activated sludge and 3,000 to 6,000 mg/L for extended aeration. Read them as a starting method rather than as a specification: the concentration your clarifier can hold without losing solids is the real limit, and it is determined on the plant rather than from a table.
How to determine it: the oxygen demand comes from the load removed and the nitrogen oxidised, then from the transfer efficiency of the diffusers at the depth of that basin, which is equipment-specific.
Why the four move together
A change to any one of the four arrives at the other three:
- Raise the sludge age and the tank has to hold more solids, at a higher mixed liquor concentration or in a larger volume.
- Raise the mixed liquor concentration and the return rate rises with it, which loads the clarifier harder.
- Lower the food to microorganism ratio and organic removal becomes more complete, but the nitrification demand appears and the air requirement can rise.
- Push any of the three beyond what the sludge can settle at, and solids leave over the weir whatever the drawing says.
The order of the work follows: the effluent requirement fixes the sludge age, the sludge age and the load fix the mass of solids, the clarifier fixes the concentration it can hold, and the mass with the concentration fixes the volume.
Common mistakes
- Sizing the tank from residence time alone, when two tanks with the same retention time hold very different quantities of biomass.
- Setting a sludge age that suits the nitrifiers at summer temperature, when the minimum nitrification age rises as the water cools.
- Treating the sludge return as a pump selection.
- Reading the ratio from the design load alone, when the operating record is the measurement.
- Assuming the plant will run at the design mixed liquor concentration when the clarifier cannot hold it.
- Leaving the clarifier out of the calculation: its surface area sets the ceiling on the concentration the train can carry.
- Copying a sludge age from a plant on different water, when age, concentration and settleability travel together.
Self-check before the numbers are fixed
- Is the effluent requirement written as what has to be removed — organics, ammonia, nitrogen, phosphorus — rather than as a process name?
- Is the design temperature the coldest the plant will operate at?
- Is the sludge age in wastewater treatment set from the slowest organism the plant must retain at that temperature?
- Is the mixed liquor concentration one the clarifier can hold, checked against the sludge volume index expected?
- Has the return activated sludge rate been derived from the target concentration and the return sludge concentration?
- Is the food to microorganism ratio in wastewater treatment calculated from the design load and the mixed liquor the tank will actually hold?
- Has the oxygen demand been built from the carbon load and the nitrogen load separately?
- Has the excess sludge production been estimated, so the wasting and dewatering duty is known?
- Is the aeration tank in a wastewater treatment plant sized from the mass of solids and the concentration together, rather than from a residence time?
From four numbers to an aeration tank
The four decisions end in a volume, a concentration and an air demand. The tank volume and HRT chart turns a residence time into a basin volume, and the plant selection and sizing guide works the same ground from design flow and discharge standard. Where the duty is small enough to be built as one package, the integrated MBR equipment page covers the membrane version of the same process. How much of your duty a single unit covers is confirmed with the factory for your design flow, influent analysis and discharge standard rather than assumed from a table.